The present invention relates generally to microfabricated signal processors, and more particularly to microfabricated signal processors which include microelectromechanical components.
The use of electromechanical filters for signal processing dates back to the second World War. An example of such early work is Robert Adler's filter (see Compact Electromechanical Filters, Electronics, 20, pp. 100-105, April 1947) which used steel plates connected by short wires to transmit vibrations and act as a high frequency filter in broadcast receivers. Mechanical filters were refined between 1950 to 1970 into effective signal processing components and were applied to a variety of systems, such as telephone channel filters.
Mechanical filters are useful in systems that demand narrow bandwidth, low loss, and good stability. Mechanical filters have excellent aging characteristics. In conventional filters, nickel-iron alloys, which are capable of quality factors from 10,000 to 25,000, are used. Piezoelectric crystals, such as are commonly used in oscillators, are also sometimes used in mechanical filters.
Mechanical filters have been displaced in audio applications with the advent of integrated switched-capacitor filters, which can be implemented with complementary metal oxide silicon (CMOS) circuitry. The decline in popularity of mechanical filters is due to their high manufacturing cost and large size. They are usually several centimeters long and about one centimeter in diameter. Furthermore, very large scale integrated circuit (VLSI) technology or modular integration of CMOS with microstructure (MICS) (see W. Yun, R. T. Howe, and P. R. Gray, "Surface micromachined, digitally force-balanced accelerometer with integrated CMOS detection circuitry", Technical Digest, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, S.C. pp. 126-131, June 1992) makes it possible to integrate the switched-capacitor filter with other functional blocks, e.g. in a one-chip modem. Standard VLSI circuitry techniques and techniques such as flip-chip multichip modulation may be used to provide what will be termed herein as "on-board" electrical circuitry.
Standard mechanical filters utilize magnetostrictive transduction between electrical and mechanical energy. Furthermore, standard mechanical filters utilize helical springs with compression applied along the longitudinal axis, or torsional springs. Fabrication of such features is problematic using VLSI microfabrication techniques since these components are nonplanar whereas these techniques produce essentially planar structures.
One of the first micromechanical filters was the resonant gate transistor developed by Westinghouse Research and Development in 1967. The resonant gate transistor is a field-effect transistor with a vibrating metal beam replacing the gate. The beam is resonated vertical to the substrate using electrostatic forces applied by an underlying electrode, and the movement of the beam controls the capacitance and therefore the charge density and conductivity of the channel. (see H. C. Nathanson, W. E. Newell, R. A. Wickstrom and J. R. Davis, "The Resonant Gate Transistor", IEEE Transactions on Electron Devices, ED-14, pp. 117-133, March 1967). Typical metal thin-film beams were 0.1 mm long and 5-10 .mu.m thick, and a typical quality factor is 500 at 5 Khz. Development of the resonant gate transistor was abandoned due to low quality factors, high temperature coefficients of the resonant frequency, poor aging characteristics of the metal films, and severe amplitude constraints on the input signal due to the use of a nonlinear electrostatic drive (see W. C. Tang, T.-C. H. Nguyen, and R. T. Howe, "Laterally Driven Polysilicon Resonator Microstructures", Sensors and Actuators, 20, pp. 25-32, 1989).
In the present invention microresonators are fabricated from polycrystalline silicon, and driven by capacitively-coupled interdigitated electrodes. The interdigitated electrodes provide a planar structure with a linear response. Polycrystalline silicon is an attractive material for manufacturing since it is easy to fabricate and provides a low-loss material. The microresonators include folded flexures which are also essentially planar and provide a large displacement with a linear spring constant. Displacement of the electrodes produces bending, rather than torsion or compression, of the elements of the flexures. The micromechanical filters may be integrated with on-board microelectronic circuitry to provide an efficient system for signal processing.
The micromechanical filters of the present invention may be implemented as series resonators with mechanical coupling, as parallel resonators with electrically summed responses, or as combinations thereof. Both approaches achieve high signal-to-noise ratios and quality factors on the order of 50,000 in vacuum. The resonant frequency of the micromechanical structure may be controlled by a trimming process after fabrication of the structural components (see T. M. Bloomstein and D. J. Ehrlich, "Laser Deposition and Etching of Three Dimensional Microstructures," Transducers '91, pp. 507-511, San Francisco, June 1991, or G. K. Fedder and R. T. Howe, "Thermal Assembly of Polysilicon Microstructures", IEEE Micro Electro Mechanical Systems Conference, Nara, Japan, pp. 51-57, January 1991). Micromechanical filters are particularly well suited for devices requiring very high quality factor circuits, such as real-time spectrum analyzers.
As shown in FIG. 1a, a micromechanical system 230 may be fabricated on a silicon wafer 240. Microresonators 232 may be coupled to on-board circuitry 234, and the micromechanical system 230 may be hermetically sealed by a shell 238. The micromechanical system 230 may be incorporated into a larger microfabricated system 250 as shown in FIG. 1b, the larger system 250 including electronic circuitry 252, 254 and 256, and interfaced to other electronic components by a plurality of leads 260.
As a single-chip component or as part of an integrated microsystem, microelectromechanical filters have potential application as IF (intermediate frequency) filters in radios and as channel filters, signaling filters, and pilot tone filters in FDM (frequency-division multiplexed) telephone systems and FSK (frequency-shift keying) modems. The ability to fabricate numerous filters of the present invention on a single chip makes these filters particularly well-suited for real-time spectrum analyzers.
An object of the present invention is to provide a microelectromechanical signal processor.
Another object of the present invention is to provide a microelectromechanical signal processor which is essentially planar and may be fabricated with VLSI fabrication techniques in a small number of processing layers.
Another object of the present invention is to provide microelectromechanical signal processors which have high signal-to-noise ratios and quality factors.
Another object of the present invention is to provide a microelectromechanical signal processor which includes many individual resonators and hence may function as a multi-channel signal processor.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.